Fix incorrect file index handling that differed between DWARF4 and
DWARF5.
DWARF4 file indices start at 1, while DWARF5 starts at 0. The code was
manually adjusting indices with `Row.File - 1`, which works for DWARF4
but breaks DWARF5.
Replace manual indexing with `getFileNameEntry()` which abstracts away
the DWARF version differences.
Fixed in:
- printDebugInfo()
- addDebugFilenameToUnit()
8c8d1d45a6